The measurement of the intraocular hydrostatic pressure is a major diagnostic tool for the identification of eye disorders, especially glaucoma, and as such, apparatus capable of accurate and reliable measurement is greatly to be desired. Tonometry is used routinely for screening the population at large as well as following individuals with known pathology: consequently, risk, inconvenience and trauma must be minimized. Moreover, of those tested, the overwhelming majority will not exhibit intraocular pressure pathology. An accurate and reliable measurement is essential to assure that the screening does indeed identify incidence of abnormal intraocular pressure while not burdening the health care system with erroneous positive identification of pathology in normal individuals. Other desiderata include safety, non-invasiveness and practice of the method without the necessity of topical anaesthetic.
In the prior art, the Goldman tonometer has been a standard for opthamalogical measurement for many years. In this approach the intraocular pressure is obtained by flattening a standard area of the cornea to conform to a planar surface placed in contact with the cornea. Applanation tonometry, as the method is known, employs a circular transparent plane surface of precisely known diameter which is urged against the anaesthetized cornea while the observer confirms the applanation condition by observing the cornea through the transparent plane surface or footplate with the aid of a small amount of flourescein in the lacrimal fluid and a slit lamp or smilar light source. The optical source is preferably rich in the blue portion of the spectrum to excite the fluorescein and thereby provide enhanced contrast. The observer adjusts the pressure applied to the foot plate until the cornea just conforms to a circular region marked on the transparent foot plate, at which point the force urging the foot plate against the cornea is recorded.
Measurements of this type suffer from error in establishing the applanation condition. This determination is subjective and prone to error arising from, among other effects, the presence of a meniscus of tear fluid at the periphery of the foot plate and the resistance of the cornea to bending. A significant source of error and difficulty arises with the length of time required to adjust the device and to observe the flattened area. This may require an interval ranging from a few seconds to a minute or more. It is difficult for the subject to maintain the eye in a fixed position for that period without blinking. Moreover, such protracted contact of the instrument with the cornea increases the risk of a scratch or other trauma because of the prolonged contact and the possiblity of gross eye movements. Clearly, the prolonged contact also requires application of an anaesthetic to the eye.
A significant improvement in applanation tonometry apparatus due to Mackay and Marg (see Marg et al, Archiv. Opthalm., v.4, no.1, pp 67-74 (1961) and references therein) utilizes electronic means to measure the force required to produce the applanation condition between the foot plate and the cornea, deriving a signal proportional to the applied force. The force is necessarily applied as a function of time and the resulting displacement of a plunger linking the cornea with the central region of the foot plate is monitored on a trace recorder. In the Mackay-Marg instrument the trace characteristically rises to a first relative maximum as the central plunger responds to the full corneal resistance and the intraocular pressure. As the cornea is applanated against the footplate region surrounding the central plunger, the corneal resistance is distributed over the annular region and the signal derived from the plunger displacement drops to a relative minimum, thereafter rising monotonically as the cornea continues to yield in response to the increasing pressure. A second maximum will be recorded when the area of the cornea applanated by the probe reaches its maximum. As the probe is withdrawn, the sequence is reversed and a near mirror image of the trace is generated. For this type instrument it has been found that the significant indicia for establishing the magnitude of the intraocular pressure is the relative amplitude of the aforementioned relative minimum or trough with respect to the baseline of the trace. The force measurement is derived from the displacement of the plunger. The displacement is quite small, of the order of a micron. The Mackay-Marg tonometer is (at least in principle) free of the need for an anaesthetic because the entire measurement is obtained in an interval of the order of 10's of milliseconds. An interpretation of the complex trace is still required for this instrument to extract the critical intraocular pressure parameter.